1. Field of the Invention
The present invention relates to a method of estimating a maximum output of a battery for a hybrid electric vehicle (HEV), and more particularly to a method of estimating an accurate maximum output of a battery for a hybrid electric vehicle depending on various environmental parameters affecting the maximum output of a battery mounted to a vehicle, thereby increasing an efficiency of the battery and preventing an over-charge or over-discharge of the battery.
2. Description of the Prior Art
In general, an electric vehicle uses a lithium-ion cell, which is a secondary cell, as driving source, drives a power generating device using the power outputted from the cell and transferring it to a driving wheel via a power transmission device to rotate the wheel, thereby driving the vehicle.
In the cell, as a discharge proceeds, a terminal voltage between a cathode and an anode is gradually decreased and reaches a predetermined limit. At this point, the voltage is suddenly dropped to reach a discharge end voltage. After that, the discharge capability is vanished. If the discharge is continued below the discharge end voltage, there occurs a chemical reaction with an electrolyte, so that an electrode plate generating current is damaged and thus a function of a storage battery is lost.
Accordingly, the electric vehicle is structured such that it travels as much as a capacity charged in the battery and re-uses the rotating power of the driving wheels during the traveling to recharge the battery. If the electric vehicle is driven even until the battery is completely discharge and thus stopped during the traveling, it is difficult to recharge the battery. Accordingly, it is important to accurately perceive a residual capacity (SOC; State of Charge) of the battery during the traveling. However, it is difficult to accurately measure the charged amount because of an irregular property of the battery (i.e., it is changed due to temperature, discharge, etc.), as compared to a fuel gauge of an existing vehicle.
Commercially, methods of checking the SOC of the battery include measurement of a cell voltage or a discharge capacity in order to check the SOC.
However, in the former method, the voltage is decreased depending on the discharge amount. In other words, the voltage is suddenly decreased in rapidly accelerating, irrespective of the SOC. In addition, in the latter method, since the available capacity of the cell is different depending on load conditions (for example, the constant speed traveling or traveling in city is made at how kilometers per hour), an algorithm for checking the SOC is very complex.
In the mean time, parameters affecting the maximum output of the battery include a charged state of the battery, a temperature environment and an output degradation depending on the discharge capacity during the traveling. In generally, the battery adopted in the hybrid vehicle undergoes the rapid charge/discharge due to the rapid acceleration and deceleration which frequently occur during the traveling. Due to the rapid charge/discharge, the charged state and the maximum output of the battery exhibit a dynamic behavior having a high rate of change. In addition, the maximum output of the battery is highly changed depending on the temperatures. Accordingly, it is very difficult to accurately predict the maximum output of the battery during the traveling due to the various parameters.